In thermal machines, efficiency which is as high as possible has always been the target in order to use the applied fuels more effectively for power generation. In the case of gas turbines, the aim is an efficiency of 63%, for example, for which higher combustion temperatures in the region of 1850K would be required. In order to achieve this, thermally highly loaded components of the machine have to be cooled by means of complex cooling devices and configurations. On account of the increasing complexity, problems in the production of such components increase and lead to high scrap rates.
In the case of gas turbines, on account of an irregular profile of the combustion chamber exit temperature, critical hot zones in the subsequently arranged components, such as stator blades or rotor blades or wall elements of the hot gas passage, occur, resulting in local overheating so that in such components working temperatures which are approximately 80-130K higher than the hot gas temperature are to be taken into consideration in the future.
For this reason, very efficient local cooling of the thermally highly loaded components is required in the case of gas turbines and comparable thermal machines.
One possible way, in which such efficient local cooling can be developed, is near-surface or near-wall cooling which is shown in two variants in FIGS. 1 and 2. The component 10′ (tubular in the example) from FIG. 1 has a wall 11 with a thickness t which is 4 mm, for example. Hot gas impinges upon the component 10′ from the outside (block arrow). Cooling medium, mostly air or steam, flows through the interior space 12 of the component 10′ and at least partially dissipates the externally introduced heat from the wall 11.
An improved alternative cooling configuration is reproduced in FIG. 2 for the component 10. In this case, parallel cooling passages 13, through which flows cooling medium, with an inside diameter d1 of 1 mm, for example, extend directly in the wall 11 and are only at a distance d2 of 0.5 mm, for example, from the outer surface of the wall 11.
A transition from the configuration in FIG. 1 to the configuration of FIG. 2 enables a reduction of the cooling medium mass flow by 40-55%, or an increase of the hot gas temperatures by 50-125K, on account of the reduced distance between cooling medium and hot gas.
Such a configuration can be achieved in components with effusion cooling in the following way: the basis is a component which according to FIG. 3 has an effusion-cooled component wall 14′ (with a thickness of 2.0 mm-5.3 mm, for example) through which oblique cooling holes 15 (with an inside diameter of 0.8 mm, for example) extend from a cool side CS of the component wall 14′ to a hot side HS, through which cooling holes cooling medium 16 flows and discharges on the thermally loaded surface 18.
In the case of a component according to FIG. 4 with a comparable wall 14, instead of cooling holes 15 cooling passages 17 are formed in the component wall 14 and with an inside diameter of 1.0 mm, for example, comprise a plurality of sections 17a, 17b and 17c. The first passage section 17a extends from the inlet on the cool side CS into the interior of the component wall 14. A second passage section 17b adjoins the first passage section 17a and (in the manner of the cooling passages 13 in FIG. 2) extends essentially parallel (at a distance of 0.6 mm, for example) to the surface 18 which is to be cooled. A third passage section 17c then adjoins the second cooling passage 17b and terminates in an outlet on the hot side HS. The first passage section 17a and the third passage section 17c are oriented obliquely to the surface 18 in this case (similar to the cooling holes 15 in FIG. 3).
A cooling configuration of the type shown in FIG. 4, as near-surface or near-wall cooling, would bring significant advantages compared with conventional cooling configurations.
Such a cooling configuration, however, poses problems with regard to the difficulties related to production engineering, which lead to high costs and high scrap rates.
It is certainly conceivable to realize such cooling configurations by casting methods in the hollow core technique. In this case, after the casting of the component the core forming the network of internal cooling passages is removed. The remaining cavities form the passages. Although this method is practical as regards production engineering, it is expensive owing to the complexity and is afflicted with high scrap rates. Furthermore, a component cannot be reworked with this technology or be subsequently altered.